A high-carbon steel sheet refers to a steel sheet that contains carbon of 0.3 wt % or more and has a crystalline structure having a pearlite crystal phase.
The high-carbon steel sheet is made have high stiffness and high hardness after experiencing the final process. Since the high-carbon steel sheet has high stiffness and high hardness as described above, the high-carbon steel sheet is used as tool steel, spring steel, or mechanical structure steel that requires high stiffness and hardness.
A method of manufacturing high-carbon steel for a spring is described below.
In order to manufacture high-carbon steel for a spring, first, high-carbon steel materials are manufactured and hot rolling, pickling and oiling line, and spheroidizing annealing processes are then performed. Next, after repeating primary cold rolling, heat treatment, and pickling and oiling line processes, the high-carbon steel for a spring is manufactured after a secondary cold-rolled process.
The reason why the pickling and oiling line process is performed after the hot rolling process is to remove an oxide layer inevitably generated in initial materials manufactured by the hot rolling process. Furthermore, the reason why the spheroidizing annealing process is performed is to homogenize a non-uniform structure of the materials resulting from the hot rolling process and also lower the stiffness of the materials so that the primary cold-rolled process is possible.
Furthermore, the primary cold rolling is previously performed in order to optimize the reduction ratio of the secondary cold-rolled process. Furthermore, the heat treatment process performed after the primary cold-rolled process is a process of determining the microstructure of the final product and is performed under proper heat treatment conditions in order to obtain desired quality.
After the heat treatment process, a pickling and oiling line process is performed again in order to remove an additional oxide layer generated in a surface of the steel materials and the final product having a desired thickness is manufactured through the secondary cold-rolled process.
The above-described method of manufacturing a high-carbon steel sheet for a spring, however, is problematic in that very high costs and a lot of time are necessary due to a cost for each process and delivery between the processes because a variety of processes have to be performed even after the hot rolling process.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.
The present invention has been made in an effort to provide a high-carbon hot-rolled steel sheet and a high-carbon cold-rolled steel sheet having an advantage of both high stiffness and high hardness by forming a fine and uniform fine pearlite structure and a method of manufacturing the same. Another embodiment of the present invention provides a method of manufacturing a high-carbon hot-rolled steel sheet from which a subsequent heat treatment process can be omitted by forming fine pearlite in a hot rolling process.